I got a quarter wave plate from Greg Ogin this afternoon. The attached plots show:

black-> RC noise when there are no quarter wave plates. It's the beat of circularly pol beams.

green-> when one wave plate is intalled.(from last week)

Blue-> two wave plates are in used. I'm surprised that nothing changed much from green.

Red-> I use the Buzby box to amplifie the demodulated signal and connect it to SR560 for filtering. The SR560 complains about overloading signal when I

set gain at 1, but it's ok when gain is 2.

It seems still far from noise budget Rana gave me. The peak around 1000 Hz might to be investigated.

 I took the data of frequency noise of the functiongenerator("Marconi") and spectrum analyzer's noise from Mott's elog on  Nov 13 14:37:11 2009,  AdhikariLab.  Thanks, Mott

 To measure the noise @160Hz, two Functiongenerators are set at 160MHz, then mix two signals togather to get phase noise. Multplying the phase noise by corresponding frequencies to get frequency noise. 

Assuming two ideal function generators, the freq noise is divided by sqrt(2) to get noise contributed by one function generator.

The attached graph shows frequency noise from Marconi noise, detector noise(spectrum analyzer that measured Marconi noise) , beat noise( noise from beating two beams after the cavities), and estimated noise.

VCO noise will be updated soon.

VCO frequency noise is measured. V input is 4.7 Volt. The signal output from the VCO (which controls the AOM) is mixed with signal from ifr2023 at ~80MHz. The demodulated signal

is then fed to SR560. Gain setting is 5 x10^3, low pass at 1Hz. The output signal is split into two. One is sent back to ifr2023 for freq modulation, another one is fed to the spectrum analyzer. The voltage output is

then converted to frequency by calibration from last week (0.714 MHz/V).

The plot shows frequncy noise from:

1: function generator (ifr2023) in brown

2: VCO,  in pink

3&4 Frank's RC noise data from FEB09, in red and blue

5:  RCnoise from beat measurment, in green

6: estimated noise

VCO seems to limit our noise at f=100Hz and higher/

I measured the VCO noise again.  2 methods that I tried

1) Measuring noise from a) suppressed signal (signal from pre-amp which is fed back to IFR2023) and b) error signal ((beat signal coming out of the mixer)) and match them together

Setup for 1)

ifr2023b,  carrier f = 79.994620 Hz, power =    7.0 dBm,     FMdevn = 500kHz

mixer     Mini Circuit ZFM-3-S+   (Lo input = 7dBm, RF input = -1.05 dBm)

pre amp, SR560, Gain inv10, low pass at 1Hz

2) Lowering the unity gain frequency and measuring only the error signal (beat signal coming out of the mixer)

Setup for 2)

ifr2023b,  carrier f = 79.995 316 Hz, power =    7.0 dBm,     FMdevn = 1kHz

mixer     Mini Circuit ZFM-3-S+   (Lo input = 7dBm, RF input = -1.05 dBm)

pre amp, SR560, Gain inv20, low pass at 0.03Hz

Converting V/rHz to f/ rHz

1)The feed back signal can be convert to freq noise by using the calibration from last week, which is 0.35 MHz/V, since two setups on IFR2023 are similar.

We can obtain the calibration by giving input voltage to the LO and see how the carrier freq changes.

2) For the  error signal out of the mixer, disconnect the feedback signal and measure the slope of the signal output. That will be [V/rad] calibration.

Then we can convert V/rHz -> rad / rHz , multiply by the corresponding f to get f/rHz

 Red and Green plot are from the 1st method,

Blue plot if from the 2nd method.

The results between two methods are not even close to each other, I'll check tomorrow to see if I did something wrong,

 I calculated the frequency for other higher Hermite-Gaussian modes, (n+m up to 20) to make sure that there is no overlap between these frequencies

and our choice of sideband frequency.

The beam frequency for higher order modes will be different from that of fundamental mode because of the phase shift. The frequency shift can be found in

Siegman, p 762 (Thanks Zach for pointing me to this). The phase shift from TEM00 will be multiples of 258.29 MHz. To find 20 possible lines, we  use

  N* 258.9  mod 737   , N = 1,2,3....20 (the free spectral range is 737 MHz), This will be the contribution from left side of the interested peak. To find the contribution from the right  side we use,

 737 -  N*258.9 mod 737.  

The plot below show the allowed frequency for higher order TEM (from the closest fundamental mode at f=0 and 737 MHz)  on x axis, the FWHM is exaggerated for clarity. They are smaller in real life.  The side bands at 21.5 and 35.5 MHz (our EOMs) are plotted in red.

I only plot from -100 to 100 MHz.

There's a correction

1) the beam waist inside the cavity, it seems that the 237 um(which corresponds to frequency shift = 258.29)  I got does not agree with the ROC and the cavity length.

 I calculated the beam waist based on the same ROC and the cavity length, and got 261 um (which correspond to frequency shift = 219.32 MHz)

This plot based on the frequency shift of 219.32 MHz step,

And now I plot from -100MHz to 100 MHz, the 35.5 MHz peak coincides with a peak from one of higher order modes( n+m = 20)

The exact number of that peak is 35.6 MHz.

Last time, the calculated beam waist I got was based on two identical mirrors with no reflecting coating layers on the back mirror. When that layers are taken into account, I got the same waist size.

The new plot is attached below. Two sidebands are 21.5 and 35.5 MHz. The peaks, from left to right, are 9th, 6th, 3rd, 28th, 25th, 22nd, and 19th order. Choosing 35.5 MHz for frequency modulation should be fine.

The layout for the new PSL setup ( lenses, and their positions are to be calculated.)

A Lightwave 100mW NPRO laser will be the source. AOM will be in the ACav path.

Two cavities will be covered by a box of insulation/ heater.

1) From the laser to the PMC,

    1/4 waveplate, to linearly polarize the elliptical polarized beam from NPRO

    1/2 wave plates and PBS, to adj the power of the beam

    lens, to focus the beam to the EOM

   two lens, two mirrors, to mode match the beam to the PMC

   a photodiode, a lens, two mirrors, (one for steering the beam, another one for attenuating the beam), for PDH locking

 a photodiode and a ccd camera, for the beam behind the PMC

* there will be a Faraday Isolator somewhere here. I forgot to add it.

2) From PMC to PBS

  a lens to focus the beam to 35.5 Mhz EOM

  1/2 wave plate, to adjust the power between two beams for  ACav and RefCav

 PBS, to split the beam into two paths

3) AOM path

 1 PBS, for reflected beam from the AOM

 a lens, an AOM, 1/4 waveplate, two irises, 1 curve mirror; to double pass the beam and shift the frequency

another iris and a mirror, to select only the 1st order beam and send it to  ACav.

4) RefCav/ACav path

 1/2 wave plate, to correct the polatization

two lens and a set of periscope, to mode match the beam to the cavity

 a pbs with 1/4 wave plate, a lens, a mirror, a photodiode, to PDH lock the beam

5) Transmitted beam

1/4 waveplate, to linearly polarize the transmitted beam

a photodiode/ a ccd camera to monitor the transmitted beam with necessary mirrors

lenses, to focus the beam to the PD that measures the beat signal

 a beam splitter, to mix two beams together

HOM frequency shift for RefCav is plotted below. The second one has clearer dots.

Y axis is the frequency shift in MHz

X axis is the (n+m)th order of the Hermite Gauss mode

The waist of the beam inside the cavity is 261 um* (symmetric cavity, R =0.5m, L = 0.2032 m.) 
Thus, the frequency shift between n+m+1 and n+m mode is 219.763 MHz. (see Lasers, p 762 for details)

Blue line represents the 0th order of the carrier's frequency (thus =0) The purple and the brown lines, at y= 35.5 and -35.5 MHz, are the 0th of + and - sidebands respectively.

The color dots represent the frequency shift from Higher order mode which is specified on x-axis, blue for HOM from the carrier's frequency, purple and brown for HOM from +/- sidebands.

 Choosing 35.5 sideband seems to be ok, the 27th order should be small and negligible. 

*The number 237 um for waist size is the effective beam waist size of the "emerging beam," not the real waist size in the cavity. The beam passes through the mirror which acts as a negative thin lens and changes the

beam parameter. 

I measured TF of PDH box, D0901351, (The one we have was modified). This box sends the signal to VCO.

SR785 measures at low frequency ( 1 Hz to 100kHz)

4935A measures at high frequency (10Hz to 1Mhz)

The integrator switch of the PDH box is turned off. This will be calculate later. The gain is set at 10.

The magnitude as mesured by 4935A is corrected for impedence match by x1.2.( 4935A and the PDH box have 50 ohm impedence for both inputs and outputs.)

This data will be used for control loop model later.

Blue, data from sr785

Green, Data from 4395A, I didnot use the power splitter to split the signal from source.

Red, Data from 4395A, with power splitter to divide power from source. (The power has to be increased to -30dB)

The first plot is magnitude of the TF, the second plot is phase shift, as usual Bode plot.

This is the control loop for the current PSL setup.

There are still components to be added.

1) TF of the PDH box, the one we have is a modified D0901351, so I measured the TF of this box when the integrator is off (April13,2010 entry.)

  This will be added in the model later. It is set to 1 for now.

2) TF of the photodiodes, I assume they are  Newfocus 1811 and choose the same value as used in linfss6.m.

3) I will verified the value of TF of the RefCav path (both Fast and PC paths are calculated from D980536) to see if they agree.

4) The TF of actuators will be added later.

I calculated the TF of the modified PDH box and fit it with the measurement. The comparison does not match perfectly. I'll take a look and check if all Rs and Cs in the circuit are actually the same as those in the box.

The circuit can be found at:

https://dcc.ligo.org/DocDB/0003/D0901351/002/pdh_b_v2.pdf

I checked only U7 and U4

R28 is 360 ohms

C18 is 3300 pF

C6 is 0.66 (2x0.33uF) uF

R30,R31, R23,R16,R24 have the same resistance as specified

C20,C28, C29, C14,C15, R25, C11 are removed.

 The calculation assumes that the integrator switch is off (R16 is connected parallel to R24 and C6)

If this works, the TF for PDH will be used in the simulink model.

    The values of some r and c in the circuit are corrected, I used wrong values last time ( details will be added later.)

   The measured TF and calculated TF using LISO are plotted below. The measurement and calculated data agree well from 1 to 10^5 Hz using SR785.

 The correction factor due to mismatch impedance when using 4395A will be checked again.

I measured the beam waist of Lightwave NPRO 1064nm 100mW with WinCamD.

The nominal beam waist are 380 um and 500 um, 5cm from the center. the number I got from the measurement are 237 um (major) and 187.3 um (minor) which are quite different from the nominal values.

I'll check it again tomorrow to see if the data are still the same.

I measured the beam waist again. The laser was operated at full power ~100mW. A mirror attenuated the beam to 60 uW and ND  4.0 was on the CCD.

The fits give

 Wx = 155 um, 3.45 cm in front of the opening.

Wy = 201 um, 2.8 cm in front of the opening.

Details for Mode Matching

1)   Laser to PMC

The laser has

              Wx = 155 um, 3.45 cm in front of the opening.

            Wy = 201 um, 2.8 cm in front of the opening.

The average number for calculation is w = 180 um, 3cm  in front of the opening.

First, we focus the beam to the EOM, w can be  250 – 500 um.

We pick 350 um.

F= 200 mm,

W1= 180 um

W2=350 um

Distance from w1 to the lens, d1, = 9.36”.

Distance from w1 to w2, L, = 22.8 “.

Then we mode match this beam to PMC

F1= 50.2mm

F2= 63mm

W1=350 um

W2= 370um

Distance from w1 to first lens, d1,         =157mm =6.2”

Distance from first lens to 2^nd lens, d2 =120mm = 4.7”

Distance from 2^nd lens to PMC, d3         = 358mm

Distance from w1 to w2, L,                      =  63.5 cm = 25”

I'll check if I can find f =200mm, 63mm, 50mm in the lab or not.

I'm trying to align the PMC.  The transducer is connected to the PMC servo card's HV out.

HV in is driven by a function generator with triangular function. The output signal looks weird. It is not a nice triangular form, it's more like a u shape waveform connecting to each others with a plateau on top.

I'll check if the transducer on the PMC and the HV out from the card are working correctly or not. Right now, there is not a glimpse of signal coming out of the PMC on CCD.

don't the words HV IN ring a bell? look into the schematic. there are only a couple of inputs at the front so it's not too hard to figure out why there was already a BNC cable connected to that input. Did you check the monitor signal? If you would you would have seen that your HV supply is missing now

After checking the PMC servo card,

ramp signal goes to ext DC 

HV in is applied properly,

HV out is fixed.

Now the PMC is scanning and working fine.

The alignment is done. Although optimization is still needed,  I can work on the rest of the setup.

*note on PMC servo card

Ratio between Voltage input (Vin), V monitor (Vmon), and High Voltage output (HVout),

HVout = 24 Vin

Vmon = 1/50 HVout

Vmon = 24/50 Vin

 see the attachment

 I made a plot for HOM from 24.5 MHz sidebands.  The 17th and 20th order are quite close to the main frequency and its sidebands.

      We were thinking about using 24.5 MHz sidebands, but it seems that 35.5 MHz is safer for the current setup.

 RA: plot deleted - put some details into the ELOG to explain your plots to prevent deletion.

HOM frequency shift for RefCav is plotted below.

Y axis is the frequency shift due to Gouy phase in MHz.

X axis is the (n+m)th order of the Hermite Gauss mode

The waist of the beam inside the cavity is 261 um* (symmetric cavity, R =0.5m, L = 0.2032 m.) 
Thus, the frequency shift between n+m+1 and n+m mode is 219.763 MHz. (see Lasers, p 762 for details)

Blue line represents the 0th order of the carrier's frequency (thus y axis =0) The purple and the brown lines, at y= 24.5 and -24.5 MHz, are the 0th of + and - sidebands respectively.

The color dots represent the frequency shift from Higher order mode which is specified on x-axis, blue for HOM from the carrier's frequency, purple and brown for HOM from +/- sidebands.

Executive summary

1. We replaced the northeast air spring on the vacuum chamber, because it was leaky.
2. We opened the transmission side of the vacuum chamber, removed the silica/tantala cavities, and inserted the AlGaAs cavities. The configuration is as follows:
   • SN 00095: south. Logo readable when standing on north side of table.
   • SN 00096: north. Logo readable when standing on north side of table.
3. We scanned the modes of the north cavity and did some rough mode-matching to TEM₀₀. All modes (including TEM₀₀) appear to be doubled. Is this birefringence?
4. We scanned the modes of the south cavity. We we able to match into TEM₍₁₀₎₀, then TEM₉₀, TEM₈₀, etc., with relative ease (albeit with the same doubling as observed in the north cavity). However, as we got closer to TEM₀₀, we noticed the presence of two bright scattering centers near the mode axis. These scattering centers appear to be hosing the buildup of the TEM₀₀ mode in the south cavity.
5. Tara thinks we cannot proceed with the south cavity as is. We'll have to take off and reclean at least one of the mirrors.

Details

At various times, we put the transmission of the north cavity on a PDA100A and monitored the voltage on a scope while sweeping the laser PZT. For the two TEM₀₀ modes of the north cavity, the observed splitting was 11.5 ms when the PZT was driven with a 4 Vpp, 5 Hz triangle wave. Tara has previously measured the south laser PZT actuation coefficient as 3.1 MHz/V (ctn:182). This gives the frequency of the splitting as 1.4 MHz. Since the expected FSR of these cavities is 4070 MHz, this corresponds to a cavity length difference of 180 pm.

The FWHMs of the two peaks (again as seen on the scope) were 1.16 ms and 1.30 ms. With the FSR given above, this gives the finesses as 29 000 and 25 000. That's higher than what should be possible given the measured transmissivities of the mirrors [we expect a finesse 2π/(300 ppm) = 21 000], but this was a quick and dirty measurement that relies on a PZT calibration that's a few years old.

Redid optical contacting on south (for a second time) to try to get rid of scattering defects.

Spacer 95: left of ATF logo is 143, right is 137. 143 is on transmission side of chamber.

We redid the mode-matching into south, and judging from CCD images it appears to be free of gross scattering effects.

In the process of moving the seismic stack around, we found that the two rubber noodles on the transmission side had fallen over (so they were being compressed transversely instead of longitudinally). We stood them upright again, but one of them broke, so we had to swap it with a spare. (We tried for a while to make a new one by coring out a cylinder, but they seem to be very brittle. Tara suspects that they're old and broken down.)

Next step is to adjust the stack as necessary to avoid reflections from the windows.

At some point I would like to do the following:

• Birefringence measurement: temporarily swap QWP before periscope with HWP, record swept transmission as a function of HWP angle
• Redo NPRO PZT calibration: record swept transmission with 14.75 MHz sidebands on, and thereby infer voltage-to-frequency coefficient

We put on the CF gasket and closed the transmission side of the chamber. Now we are pumping down.

Tara did some work last night to ensure that the window reflections on the input side of the chamber are not overlapping with the cavity reflections. The south window reflection appears to be clipping on the bottom periscope mirror, but we can fix this later.

Next steps:

• Mode matching (including adjustment of the input lenses)
• Locking
• Realignment of transmission optics
• Re-establishing beat

Tara added some more juice to the north cavity heater last night. Now we can lock both cavities to TEM₀₀ and get a beat within the bandwidth of the 1811.

• North laser slow: 5.020 V
• South laser slow: 0.722 V
• Beat frequency: 49.3 MHz

Beat frequency drifted to 61 MHz over the course of a few hours. We need to wait for the cavity temperatures to settle.

I improved the mode-matching a little bit on the south cavity; it's about 50% (the theoretical max is 71%). The south lenses are now on translation stages.

I've attached a beat spectrum. Nothing is floated, RAM is not optimized, etc.; this is just a rough indicator of where things stand.

Here is what I think should happen next, in rough order of importance:

1. Float chamber
2. Measure RIN
3. Measure photothermal TF (I also need to recheck my photothermal code — I don't believe the coating TE part)
4. Put photothermal noise on noise budget
5. Reduce RAM.
6. Measure residual frequency noise.
7. Measure PLL noise. Use ATF DAQ and make spectral histogram.
8. Measure seismic noise (with Guralp or T240), with table floated and unfloated. Use ATF DAQ and make spectral histogram.

With the cavities locked, Tara and I took OLTFs of the PDH loops.

Below 100 kHz, we used the SR785 with a 70.7 mVpk excitation. Above 30 kHz, we used the HP4395A with a 22.4 mVrms excitation (these are the "HF" traces on the attached plot).

Before taking these TFs, we turned the loop gains up as high as possible without making the loops saturate.

• For north, the gains were 900 common and 900 fast, and the incident power was 1.26(5) mW.
• For south, the gains were 632 common and 770 fast, and the incident power was 1.05(2) mW.

We injected the excitation on common EXC. We then measured the TF which takes common OUT2 to common OUT1. This is almost the OLTF of the loop, except for an AD829 between OUT2 and OUT1 which has a gain of −4 V/V.

Currently I'm not sure how to explain the magnitude discrepancy between the SR785 and HP4395 measurements. Both OUT1 and OUT2 have a 50 Ω output impedance, so I would expect the impedance difference between the SR785 and HP4395 would cancel out in this measurement.

Tara and I took an SR785 measurement of the north photothermal transfer function.

Clearly there's something wrong with the measurement above 1 kHz.

Tara and I took another photothermal TF of the north cavity today. Relevant parameters:

• Power incident on cavity: 10 mW (up from the usual 1 mW)
• Beat frequency: 1.2 MHz, drifting to 650 kHz (we are hoping it will swing through 0 Hz overnight and settle above a few megahertz by tomorrow)
• DC voltage on north ISS PD: 2.39(5) V
• DC power transmitted through cavity: 3.77(2) mW
• PLL actuation coefficient: 50 kHz_pk / 1 V_rms
• PLL UGF: 80 kHz (measured)
• EOAM drive: 5 Vpp from 20 kHz to 300 Hz, then 3 Vpp from 300 Hz to 0.2 Hz

In the attached data I have already converted the raw data (in V/V) into hertz of beat frequency per watt of circulating power. For this I use the conversion factor (50 kHz / 2^1/2 V) × (2.39 V / 3.77 mW) × π / F, with F = 16 700. Since the TF (again) appears to be junk above 1 kHz, I haven't bothered undoing the CLTF of the PLL.

The attached plot shows the expected photothermal TF in terms of hertz of beat frequency per watt of absorbed power per mirror. Therefore, the scaling factor that makes our measurement (given in hertz per watt of circulating power) overlap with the expected TF (given in hertz per watt of absorbed power per mirror) should be the average absorption of each mirror. I find that this scaling factor is 6 ppm, which seems surprisingly low, especially given our earlier finding that we have at least 120 ppm of scatter + absorption loss. So I will double check for missing factors of 2, 4, π, etc.

At any rate, the shape of the measured transfer function appears to be in good agreement with the expectation up to 100 Hz. If we believe that the coating/substrate photothermal crossover happens around 10 Hz, and we believe our measurement from 10 Hz to 100 Hz, then this seems to indicate that the thermo-optic cancellation has been somewhat successful.

Some minor maintenance/improvements to the optical setup:

• We replaced the existing lens before the beat PD (RoC = 51.5 mm) with a slightly faster lens (RoC = 38 mm) in order to reduce the spot size on the diode.
• Tara improved the clamping of mode-matching lenses before the south cavity (they weren't tightened down enough before)

We locked the south cavity using the north TTFSS. 200 kHz oscillation is still present, so whatever this is probably doesn't reside in the TTFSS box.

Tara and I also took cavity pole measurements using the EOAM and two PDA10CSs, one placed before the cavity (monitoring the light rejected from the post-EOAM PBS) and one after the cavity (on the ISS breadboard). The HP4395A was used to drive the EOAM, and we then took the transfer function which takes the pre-cavity PD voltage to the transmission PD voltage. I will fit these later, but the poles appear to be consistent with the values tabulated in ctn:1475.

We turned on both ISS loops today.

Here is an in-loop characterization of the south RIN with and without the ISS.

we replaced the photodiode in the RFPF fro the reference cavity (RCAV). The old one looked like shit. Below pictures of the old and new photodiode.

 old photodiode - picture 1

old photodiode - closeup view

new photodiode

The TF of 2 35.5MHz RFPD and 21.5 MHz RFPD are measured by the Jenne laser. RCAV's RFPD has the better response, while ACAV's RFPD peak is slightly shift to 37 MHz. But the peak is low Q so we lose only a few dB. PMC's RFPD is now tune to have a resonance peak at 21.5 MHz and a notch at 43 MHz as it should be.

cavity is back in chamber and started pumping. Had some trouble with one of the windows. More details and photos later...

Front and back Periscope for RCAV are back on the table. The input beam is aligned.

The transmitted beam is coarsely aligned.

Note that the coupling efficiency for RCAV is down to ~ 85%, probably from the change of cavity's position.

I used the periscope to adjust only beam's position and angle. I haven't tried to reposition the lenses yet.

The input beam has multiple reflections from the periscope's top mirror, but beam is quite center on the mirror, not sure what causes this.

 Instead of a single beam, I get one bright center beam and top and bottom faint beams ??

The cavities' insulation is back on both cavity. However, the outer box front panel is left open for the vacuum pumping.

We should be able to measure the beat by tomorrow.

We measured beat noise. With new suspension, the signal is getting worse from ,probably, seismic  .

The peak at 3.73 Hz is suppressed, but other mechanical peaks appear.

RCAV is aligned, the efficiency is ~98%

The plot below shows the beat and seismic measurements (x, y, z directions) in arbitrary unit.

The seismometer is placed on the optical table

These peaks have not been seen before, so it seems that our new suspension is not working well, especially

on vertical (z) direction.

We will investigate what cause those peaks, and see if we can fix them.

we moved a nitrogen bottle from the tiltmeter lab to the CTNlab to float the table. Last time i tried this we realized that the pressure of the compressed air supply in the lab is not high enough (~35PSI max). As we now know that we need more seismic isolation we wanted to give it another try to see how much gain from floating the table and to determine how much we have to change the actual cavity support.

After connecting the table to the bottle and trying to adjust the regulators on the legs we saw that three legs seem to work fine but the forth one didn't move at all and one could hear that some air was leaking somewhere from inside the top of the leg, which we couldn't further investigate (no space when attached to the table). However the regulator seem to work just fine. So we decided to replace that leg. We took the pallet jack from the 40m to lift the table. After replacing the leg and adjusting the valves the table is now floating.

The maximum pressure for the legs is specified as 100PSI. In order to float the table we need about 90PSI, at 80PSI only part of the table floats. So we have to stick to the nitrogen bottle (or compressed air). We have a bottle rack so it's not a problem. We just have to run a hose to there which we have to buy. The hose has an O.D. of 1/4" and less than .17" I.D. We got the Guralp from the 40m to measure the isolation we gain from floating the table which we will do tomorrow.

.

We measured seismic noise (on three directions) on the table when the table was floated and not floated. Seismic is substantially reduced at frequency 10 Hz and above.

Note: the data is calibrated to velocity/rtHz. ( I checked the 40m entry, I used this calibration here. The data was corrected for 50Hz pole as well.

(after calibrating, the result look similar to what Jan did last year, see here psl:435)

==guralp cmg-40T setup==

We used Guralp CMG-40T 3-axis seismometer (borrowed from 40m with Jenne's help) to measure the noise on the table. The setup on the seismometer is "Broadband velocity". Signals from "Low velocity" channels are used to acquire the data.

Voltage supply is 12V, the output ground of the box is fake ground and it should not be connected to the ground of the oscilloscope/ spectrum analyzer. I used float ground on the spectrum analyzer, so it should be ok.

The voltage output for each channel is +/- 10V.

The position was tuned by adjusting the legs of the seismometer, so that the position readout for each channel is close to zero (we got less than +/-0.3V), the manual says it should be below 3.5V, so we are ok.

==seismic improvement==

3 directions and the corresponding resonant frequencies are

1. Vertical    (2.5 Hz)
   
2. North-South: horizontal transverse motion of the cavity (normal to cavity's beam line)  (1.8 Hz)
   
3.  East-West:  beam line direction       (1.7 Hz)

The seismometer has flat response up to 50Hz, the data has been corrected for the transfer function of the seismometer.

***The noise is getting worse at low frequency below 3 Hz is typical [link to Newport], and the signal above 50Hz are mostly noise, so both signals from floated and unfloated table are similar. (2011/10/06)

Transfer function from the floor to floated table. [Newport]

===beat improvement===

Then we checked the beat signal. We just made a quick check to see if the peaks between 10-100Hz would change or not, so we did not try to optimize the loop or anything. The result (red) improves, those peaks are decreased significantly, but it is bad that noise at frequency above 100 Hz goes up a little bit. This might be from the un optimized servo. So, the next step will be checking the servo, noise at high frequency.

we have replaced the second leg which was leaking. It could be that the legs are simply to old and the rubber got brittle. As far as i know the table has never been operated floating as it had the suspended reference cavity on it since the beginning. We operate at around 85 PSI, maximum operational pressure is specified at 100PSI so that should be OK. The second leg started leaking after one day of floating operation. We disassembled one today to have a close look but we can't really tell where the leak is. We will check with some pressure to see what's broken within the next few days. Let's see what the other legs do in the near future. We still have plenty of "spares", as Aidan bought two new sets (taller ones) for the old tables in the TCS lab. So we have 8 short ones which are currently not used (and seem to be newer than the ones we currently have). And they still work as passive legs.

As the table was floated, we measured the noise from error point again.

     We tried to determine if the noise bump we saw were from the window, so we place an extra window in front of a mirror [add fig] and compare to the noise when there was no window. The results are not different that much.

     From a quick look, by adjusting the input power, from 1mW to 10mW. The shape of the noise from error point changes substantially. This could be come from RFAM or scattering.  I'll measure the noise vs input power after I optimize everything first. RFAM, beam splitter, back reflection have to be optimized.

After optimized everything, I repeated the measurement that was done in this entry (noise at detection point). There is some improvement, the result is shown below.

==What have been done==

By "optimize everything", I meant:

1. The Faraday Isolator was installed back in the setup, and optimized for maximum isolation. [add pic]
2. Beam splitter and quarter wave plate sets (for double passed AOM and cavities) were optimized for minimum back reflection.
3. EOM with half wave plate were adjusted for minimum RFAM (reduced by ~ 20 dB)
4. Beam alignment to the cavities: visibility are up to 93% for both cavities now.

 Measurement Recap:  We want to check the noise from scattering noise or RFAM at the detection point, so we measure the noise at error point when the beam reflected off the cavity, or a mirror in front of the chamber (cf entry:700). We also want to see the dependent on power input, so we chose 10mW, 5mW and 1mW input power.

    Below are measurements from error point (Mixer out) which are calibrated to absolute frequency noise through the slope of error signal from each setup (power input of 10mW, 5mW and 1mW)

==Comments for the results==

1.    The noise level when the beam reflected off the cavity goes down when we float the table and optimize everything ( red, green, blue are lower than pale pink (result from 2011_10_07).
2.    The noise level when the beam reflected off the cavity do not change much with input power ( red, green, blue are about the same)
3.    At 1mW the noise at high frequency (above 50Hz)  raises up for both cases (reflected off cavity / mirror) I'm not sure what happens. The calibration seems to be ok since the noise level at low frequency matches the results from 5mW and 10mW setup.
4.    Seismic stack's resonant frequency at 6.7 Hz shows up more clearly after RFAM/ back reflection are minimized.
   

To Do: We will check the beat signal tomorrow. There should be improvement, since optics are optimized and the table is floated.

I think another leg, or both of the old legs have leakage. The nitrogen tank is now empty.  So the latest beat signal (in blue) here was measured when the table was not floated, but the optics are optimized.

I also tried to lock the cavity with both servos (old fss and TTFSS), The beat signals look similar (the result was not saved, only through inspection by eyes), so either noise from detection point (RCAV RFPD) or noise from ACAV loop might be the limiting source at high frequency.

 == beat signal after optic optimization==

           Fig1: Current beat signal (blue), the table was not floated.

== RFPD==

As a quick test, we checked the RFPD, by measuring the in loop noise at error point (MIXER OUT) with the current RCAV's RFPD. Then compare the results with the RFPD Raphael modified (see here). Although we don't know the performance of the current RFPD, we expected them to be somewhat different.

I used old FSS for this test, input power is 2mW, Common gain = 30 (max), Fast gain =15.6.  Higher Fast gain causes the system to be unstable.

Result 1: The noise level at error point: The noise level is ~ 100 nV/rtHz. at high frequency. Not particularly good. Both RFPDs give very same noise level.

Result2: Noise at error point calibrated to absolute frequency noise, compared with beat signal.

As both RFPDs give the same noise level at the error point, this should mean that the gain of both RFPDs are the same, and the calibration factor to absolute frequency will be similar in both cases. I measured the error signal slopes to be 3.6 e5 [Hz/V] and 2.9 e5 [Hz/V] for current and modified RFPD respectively. I decided to use measured calibrations from each case in the plot so that the level of noise due to possible error bar from the measurement can be seen. The level of error point noise is quite close to the beat level (a factor of ~1.4 - 1.6)

This means that we might be limited by RFPD noise at high frequency.  It might be something else. More investigation will be done. (I did not see if the noise level change with the gain or not at the current setup, I just cranked the gain up to what ever it was before it started to oscillate. If the noise still goes up/down with the gain, it's not RFPD noise)

 Today we measured frequency response between beat signal and seismic noise, and coherence when the table was not floated. The data are calibrated based on the information from 40m:5196 (with gain 1 for out setup).(The plot is not calibrated yet)

 ==Motivation==

     We wanted to learn more about how seismic couples to beat measurement for the current setup. So measuring the frequency response between beat signal and seismic should tell us about the TF between the table to the cavities. For example, we can put a noise budget due to seismic when the table is floated by measuring the seismic on the floated table, then multiply by the TF we measure, plot it on the noise budget.

==setup & calibration==

    Details about the seismometer we used can be found here psl:696.  The table was not floated. The direction and positions of the seismometer (pink circles) are shown below in the map. All the results in this entry come from position 1 only. We checked that the signals of seismometer did not change much among each position.

Signal from seismometer was sent to SR785 chA. Signal from beat was sent to chB. We used Frequency response measurement (B/A) with vector average. The TF between beat and seismic noise from 3 directions were recorded along with coherence between them.

The beat signal has calibration = 70 kHz/Volt. Seismometer has calibration of 800 [Volt/velocity] or [Volt *sec]/ [meter].

The result (recorded in dB [V/V]) can be converted to [Hz / (m/s) ] by 10 ^dB/20 x 70 k [Hz/Volt] x800 [Volt / (m/s)]

==Results==

The top panel shows the magnitude of the transfer function. Phase is shown in the middle panel. The bottom panel presents the coherence between seismic and beat for each direction. We are quite surprised that there is no strong peak around 6-7 Hz which is the resonant peak of the stack motion. We certainly see the peak around 16Hz which probably comes from bounce mode of the stack.

as we only did a quick-and-dirty measurement of the spring constant last time we re-measured it a couple of times. The resonance frequency varies by a few Hz from measurement to measurement and it also depends on over how many periods (and where, beginning or more to the end) of the ring down we measure. So we will analyze some ring down measurements using the same setup (Al-block with accelerometer resting on one RTV spring sitting on the optical table) and take the average for a new calculation.

list of finished items: 

• in-vacuum cables and connectors, screws, washers, teflon pieces, sensors etc cleaned - baking over night
• cut & cleaned RTV springs - baking over night
• polished and cleaned second shield
• measured spring constant for all RTV springs to compare later with stack TF - see next elog entry for details
• got lots of parts from the machine shop (periscope, pd mount, parts for beat setup - all parts cleaned and ready for assembly

all parts needed for upgrading the stack and adding radiative shields/heaters are tested/ready or currently beeing baked. Installation can start Wed/Thu

unfinished things left for tomorrow:

• replace leg
• replace opamp in PDHbox

We measured Q and k of RTV springs. Currently, there are 9 of them. The results are ok

An aluminum block with accelerometer (0.398kg) was placed on a spring. The block was tapped, and the ringdown response was measured.

* the piece is broken, so it is softer and more lossy.

** the piece is made of other material.

Mean Q (piece 1 to 7) = 13.29 +/- 1.11

Mean k (piece 1 to 7) = 21,286 +/- 1,500

we replaced the mount for the combining beam splitter in the beat setup as it caused a large, broadband peak in the spectrum around 1.4kHz. The new mount is one of the old, fixed turning mirror blocks they used in initial LIGO at LLO as far as i know. After replacing the mount the peak is entirely gone. I've used two springs instead of one to increase the pressure.  We could not determine the resonance frequency of the new mount. Tapping the mount excites only known mechanical resonances from the surrounding mirror mounts. Tara posted a plot for comparison before and after replacing that mount (see here). He also has prepared a nice plot combined with a drawing which mount corresponds to which resonance we see in the spectrum. We will use this to start reducing (or even eliminating) those resonances starting with the most dominant ones close to 1kHz

Attached a copy of the drawing.

we've installed:

• the new stack -see earlier post with measured TF
• both copper radiation shields
  - one shield has a NiChrome heater wrapped around (the other one not as we first want to see if we need more/less power, but calculation says this one should be right)
  - three temp sensors (AD590), two on the shield with heater, one on the one without
  - those sensors won't be used for temp control, only for temp and gradient monitoring and/or safety shutdown (if required)
• cable from D-SUB feedthrough to rack breakout terminal

window is back on and pumping down over the weekend. Thermal insulation for vacuum chamber is back in place, so we should be ready to go for a new measurement on Monday afternoon after installation of the beat breadboard

We've installed two pd's, one for each cavity. As the demodulated signal was very small when we optimize the EOM the usual way so we decided to replace one PD with a 14.75MHz resonant PD we've stolen from the TNI to get some more signal. The other one is a 10CS from Thorlabs (which i wanted to use for both of them). It actually turns out that we don't gain very much using the resonant PD, so we might switch back to the other one.

The DC level of the demodulated signal is less than 1mV, the noise level is limited by the readout electronics when optimized (nV level). We currently have one SR560 (DC-coupled) each with a gain of 1000 and sitting on the white noise floor. We will check with a better pre-amp tomorrow.

We temporarily sample the signal taken in front of RCAV with EPICS to monitor it over time. The EOM is not stabilized, only passive isolation is installed. When we misalign the EOM and start heating the EOM we can see the noise spectrum going up and down periodically, as well as the (tiny) dc signal, so everything is as expected, except that we can't measure anything when optimized.

schematic, pictures,diagram, etc  later when we have a final setup.